burstone

/ˈbɜrstoʊn/
noun
  1. A hard, fine-grained stone used for grinding grain or sharpening tools; also called buhrstone.
    • The old mill used a large burstone to grind wheat into flour.
    • Burstone was prized for its durability and even texture in milling.
    • He sharpened his knife on a burstone until the blade was razor-thin.
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